Phenylene polymers and copolymers exhibit a number of desirable properties including high strength and stiffness, solvent and corrosion resistance, and high use temperature. The polymers often have rigid backbone structures and can therefore be used to produce molecular composites as well. Polyphenylenes of significant molecular weight are exceedingly difficult to synthesize because the growing polymer molecules lose solubility and precipitate from the reaction solvent at a low degree of polymerization (DP).
A desired increase in the solubility of polyphenylenes has been achieved by incorporating pendant side groups on phenyl-based monomers, for example, as disclosed in U.S. Pat. Nos. 5,886,130; 5,227,457; 5,824,744; 5,830,945; and 5,976,437. Substituted polyphenylenes produced in accordance with the teachings of the above referenced patents have demonstrated many desirable mechanical properties.
The coupling of aryl compounds to form biaryl compounds or polyaryl compounds via carbon-carbon bonds is of great synthetic importance. A large number of methods are known to effect such couplings, including Ullman couplings of aryl iodides and bromides (see P. E. Fanta, “The Ullman Synthesis of Biaryls,” Synthesis, 9, 9-21, 1974), coupling of aryl bromides and iodides with aryl boronic acids and esters using palladium catalysts (A. Suzuki, Acc. Chem. Res., 15, 178, 1982), reductive coupling of aryl halides with magnesium via Grignard reagents using nickel catalysts (T. Yamamoto and A. Yamamoto, Chem. Lett., 353-356, 1977), reductive coupling of aryl chlorides with zinc using nickel triphenylphosphine catalysts (I. Colon and D. R. Kelsey, J. Org. Chem., 51, 2627-2637, 1986; and U.S. Pat. No. 4,326,989) and oxidative coupling of phenols using iron (III) or air and copper catalysts (L. F. Fieser and M. Fieser, Reagents for Organic Synthesis, Vol. 1, 390, 1967).
Several reaction methods may be used to prepare substituted polyphenylenes via aryl coupling. The simplest rely on reductive condensation of 1,4-dihaloaromatics, either by way of a Grignard reagent, or directly in the presence of a reducing agent such as zinc metal. A catalyst, such as bis(triphenylphosphine) nickel (II) chloride or 1,4-dichloro-2-butene is used. Para-bromoaryl boronic acids may be coupled using palladium based catalysts. Polyphenylenes have also been prepared by methods which do not give exclusive para linkage, such as Diels-Alder condensation of bis-acetylenes and bis-pyrones, polymerization of 1,3-cyclohexadiene followed by aromatization, and oxidative polymerization of benzene.
Thus, a number of possible methods exist for the production of substituted polyphenylenes. The core technology for commercial polyphenylene synthesis is the metal catalyzed coupling of dihaloaryl species. For example, nickel-catalyzed coupling reactions have been described in several U.S. patents, including U.S. Pat. Nos. 5,227,457; 5,886,130; and 5,824,744; the disclosures of which are incorporated fully herein by reference.
Generally, this method uses a nickel catalyst to couple dihaloaryl species in conjunction with a triphenylphosphine (TPP) ligand and a zinc metal reducing agent in a polar aprotic solvent such as N,N-dimethyl acetamide DMAc or (NMP) N-methylpyrolidone. Such a reaction can be diagrammed as follows, where Y is a substituent and X is a halogen:
Although the above diagrammed method can produce commercial quantities of substituted polyphenylenes, several aspects of the process are undesirable from synthetic, manufacturing, and environmental standpoints.
First, in the nickel catalyzed process the reduction is heterogeneous in nature, involving a solid-liquid interface, where solid zinc particles must act as the reducing agents for the solvent based reaction. This process effectively renders the extent and rate of the reaction subject to factors such as zinc particle size, shape, and quality, which are difficult to control and monitor.
Second, commercially available zinc particles are partially coated with zinc oxide (ZnO), which must be removed to activate the zinc so that the substitution reaction can proceed efficiently. The zinc activation process has a number of drawbacks. For example, mechanical removal of the ZnO coating leaves ZnO particles behind as an impurity, and chemical removal results in various byproducts. For instance, one zinc activation method utilizes hydrochloric acid (HCl), which is highly corrosive and toxic and produces explosive hydrogen gas as a by-product.
Third, nickel itself is a known carcinogen, forcing manufacturers to follow a number of expensive and time-consuming environmental and safety regulations.
Fourth, the catalyst package requires the use of a substantial excess of the TPP ligand with respect to the catalyst for the reaction to proceed efficiently. TPP is expensive and is presently unrecoverable; thus, increasing the cost of the process.
Fifth, the reaction is very water sensitive. For example, generally acceptable amounts of water are typically below 50 ppm. But, the polar aprotic solvents used in the process are highly hygroscopic creating substantial manufacturing and operating challenges.
Although not directed to producing substituted polyphenylenes, a number of recent studies have suggested novel pathways of coupling aryl compounds to form biaryls. Examples include publications to Mukhopadhyay, et al. (J. Chem. Soc., Perkin Trans. 2, 1999, 2481-2484); (Organic Process Research and Development, 7, 2003, 641); (Tetrahedron, 55, 1999, 14763); and (J. Chem. Soc., Perkin Trans., 2, 2000, 1809-1812), the disclosures of which are incorporated herein by reference.
Broadly, these new pathways employ supported metal catalysts such as palladium on carbon substrate, Pd(C), or rhodium on carbon substrate, Rh(C), to accomplish aryl-aryl coupling. In such pathways the starting materials are still haloaryls, and a number of common reducing agents have been employed for catalyst regeneration, including, for example, zinc, formate-salts, and hydrogen gas. The solid-liquid reaction is often assisted by a phase transfer catalyst such as polyethylene glycol (PEG). Significantly, these reactions are generally tolerant of water and air allowing less stringent reaction conditions, and may optionally be conducted in waterborne systems.
Reactions involving heterogeneous catalysts, for example, palladium on carbon, Pd(C), have been employed to produce biphenyls from monohaloaryl molecules. Unfortunately, conversion of the monohaloaryl species in these reactions leads to the uncontrolled production of two products; 1) the desirable coupling of haloaryl molecules, for example, two chlorobenzene molecules couple to form a biphenyl molecule, and 2) the undesirable reduction of the haloaryl species, for example, chlorobenzene being reduced to benzene. A representative reaction scheme showing both the desired coupling product and the undesired reduction product is given below, where X represents a halogen and Y represents one or more substituents.

In principle, supported metal catalysts such as Pd(C) and Rh(C) could be used to produce substituted polyphenylenes from corresponding dihalobenzenes. In order for such a step-growth synthesis to produce polymers having sufficiently high molecular weight to exhibit useful mechanical properties, the coupling efficiency of the reaction has to be vastly predominant over the reduction of the functional halo substituents. Otherwise, the reaction terminates (by reduction) before the growing polymer chains have become long enough to impart useful properties to the product material.
For example, the highest reported coupling efficiency of the Pd(C) or Rh(C) catalyzed process is 93%. Consequently, in such a reaction fully 7% of the monohaloaryl species is reduced. This degree of coupling among dihaloaryl molecules would correspond to an average degree of polymerization (Dp) of only 10 to 20. However, optimal properties of polyphenylenes are only obtained when the Dp is greater than 50, and preferably greater than 100. Therefore, based on the results reported in the literature, heterogeneous metal catalysts appear insufficient to produce high molecular weight substituted polyphenylenes.
Accordingly, a need exists for a new, efficient, environmentally friendly, cost-effective method for the production of substituted polyphenylenes.